Goldenrods for the Garden Richard G

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Issue 15, 2000 Plant Evaluation Notes An Evaluation Report of Goldenrods for the Garden Richard G. Hawke, Manager of Plant Evaluation Programs

ach autumn goldenrods adorn road- goldenrods are often more desirable because areas. Woodland species like wreath golden- sides, native landscapes and gardens species that spread by rhizomes can dominate rod (Solidago caesia) and zigzag goldenrod Eacross North America. For gardeners, the landscape and kill valuable garden plants. (Solidago flexicaulis) are great additions to the these bright flowers signal the end of summer A rhizomatous species may require division shade garden, while stiff goldenrod (Solidago and the beginning of the cool, sunny days of every three years to keep the plants in rigida) and wrinkled goldenrod (Solidago autumn. Goldenrods are an essential part of check. Clump-forming species grow at a mod- rugosa) make perfect choices for the the autumn landscape and provide valuable erate pace and should not require division sunny border. Goldenrods combine well late-season color in the garden. for many years. with other perennials including Joe-pye weeds, There are almost 100 species of Solidago Knowing the native habitat of a golden- asters, coneflowers, gayfeathers, buddlejas in the world, mostly native to North America. rod can help in successfully cultivating it in and grasses. The multitude of goldenrods makes it an easi- the garden. Solidago species are found grow- The interest in goldenrod as a garden ly recognized group of plants, although distin- ing naturally in such diverse habitats as open plant has increased over the past 10 years due guishing one species from another is not fields, meadows and prairies; woodlands and in part to the shorter hybrids that were devel- always an easy task. Complicating matters is thickets; and swamps, bogs and shorelines. oped for the cut-flower industry in Europe the fact that many species hybridize naturally, Most goldenrods require full sun for best beginning in the 1950s. Today the garden creating intermediate forms that are not readi- growth, although woodland species require hybrids are more important in cultivation than ly identifiable. The composite flowers of some shade. They thrive in average garden any of the species. Recent American introduc- Solidago, made up of ray- and disk-florets, are soils but are adaptable to poorer soils, and tions like Solidago rugosa ‘Fireworks’ and similar to other members of Asteraceae, the many are drought-tolerant once established. Solidago sphacelata ‘Golden Fleece’ contin- aster family. Goldenrod flowers are clustered Plants may become rampant when grown in ue to renew the gardener’s interest in golden- in panicles, racemes or corymbs, and except rich garden soils. rods. Fascination with native wildflowers and for the white ray-florets of Solidago bicolor Most goldenrods bloom from midsum- the desire to create habitats for are yellow or gold. mer to autumn, providing a good late-season wildlife also contribute to a further apprecia- Goldenrods range in size from under a display and food for birds and insects. Many tion of goldenrods. of the taller species may be “too wild” for the foot to more than 6 feet in height, and are The Evaluation Project average gardener, but there are many shorter herbaceous perennials that grow from either The Chicago Botanic Garden (USDA species and garden hybrids for sunny or shady thick crowns or rhizomes. The clump-forming Hardiness Zone 5b, AHS Plant Heat-Zone 5) began a five-year comparative study of Solidago in 1993. The project goal was to observe ornamental traits, disease and pest

Jenny Lee Jenny resistance and cultural adaptability of 25 species and garden hybrids, and to determine the best goldenrods for Midwest gardens. The inventory included taxa that were readily available at the start of the project. Twenty-two taxa were evaluated in a pri- mary site that normally received 10 hours of full sun during the growing season and was openly exposed to wind in all directions. The clay-loam soil was well-drained, was amended with composted leaves and had an average pH of 7.4 throughout the evaluation term. Turf grass pathways surrounded the beds on all sides; and the plots, each comprised of 16 plants, were separated within the beds by mulched strips. The secondary site was located in a mixed landscape where all plants received natural, partial shade from nearby trees Solidago rugosa ‘Fireworks’ 2 Plant Evaluation Notes

throughout most of the day. The clay-loam health and vigor of some goldenrods, result- present at the base of the stems, but soil was well-drained, was amended with ing in an unfavorable display. The diverse ‘Fireworks’ usually had the least amount of organic matter and had an average pH of 7.5. native habitats of the species under evaluation leaf loss of any goldenrod. Powdery mildew All plants were sheltered from wind by fences were taken into account when monitoring the was observed in 1994, 1996 and 1997, but less and a collection of shrubs and trees. Three plants for cultural adaptability to an average than 20% of the foliage was affected. taxa were grown here, including Solidago garden soil. Plant traits and evaluation The garden hybrids ‘Baby Sun’ and caesia, Solidago flexicaulis ‘Variegata’ and specifics are shown in Table 1. A summary rat- ‘Goldkind’ had similar qualities and were for Solidago roanensis. ing was assigned to each taxon based on that reason easily compared. Both hybrids had Maintenance practices in both plots were flower coverage, plant habit and health, dis- upright, tight habits early in the summer. The kept to a minimum to simulate home garden ease and pest resistance and winter injury. stems relaxed, but rarely flopped, after flow- culture. Water was provided as needed, and no A four-star rating signifies a good perfor- ering ended. Flower coverage was high on fertilizer was applied. Spent flowers were not mance, whereas a one-star rating indicates a both hybrids, but the brown, spent flowers removed, and stems were not regularly cut very poor performance. Only Solidago bicolor, detracted from the ornamental display. Brown back during the season. A mulch consisting of Solidago glomerata and Solidago macrophyl- stems and spent flowers were occasionally shredded leaves and wood chips was placed la did not complete three or more years of the removed to improve the general appearance of around the plants for aesthetic purposes, water evaluation term. the plants. Minor levels of powdery mildew conservation and weed suppression. The best overall rating was received by were noted in all years, and rust was observed Solidago rugosa ‘Fireworks’. Diminutive yel- on ‘Baby Sun’ in 1993 only. The lower leaves Observations low flowers were borne on curved, threadlike were shriveled on both cultivars by late sum- The goldenrods were evaluated for 1) flo- panicles, creating the effect of fireworks begin- mer each year. ral display, including flower color, flower cov- ning in mid-September, and attracting many The gold-mottled leaves of Solidago flex- erage and bloom period; 2) habit display, Monarch butterflies. The shrublike form was icaulis ‘Variegata’ brightened the shade gar- including height and width measurements, ornamental throughout the season, with arch- den in early summer, several months before spreading potential, habit quality and foliage ing stems, tight crowns and a slowly spreading the flowers developed. The irregular variega- quality; 3) winter hardiness; 4) cultural adapt- habit. The sturdy stems were not pulled down tion ranged from speckles to streaks, but was ability; and 5) disease and pest resistance. by the extra weight of flowers or overhead irri- always strongest on the spring and early sum- Disease-free leaves and upright stems were gation. The fine-textured leaves emerged bur- mer foliage. Variegation was usually weak by especially important since flowering did not gundy in spring and were dark green in sum- late August when flowering began. The sulfur- typically begin until August. Rust and pow- mer. A small number of brown leaves were yellow flowers were borne in the upper axils dery mildew contributed to the decline in

Table 1: Plant Characteristics and Performance Summary Ratings Overall Solidago Flower Bloom Flower Height Width Powdery Rust3 Winter Rating Color Coverage Period Mildew2 Injury ★★ altissima yellow 40 to 60% early August - early September 59 in. spreading moderate moderate yes ★★★★ 'Baby Sun' yellow 80 to 100% early July - mid August 27 in. 30 in. minor minor no ★★★ caesia sulfur-yellow 40 to 60% late September - mid October 28 in. 26 in. none none yes ★★ canadensis yellow 40 to 60% mid September - late October 76 in. spreading severe minor no ★★★ 'Crown of Rays' (Strahlenkrone) bright yellow 60 to 80% late July - September 27 in. 29 in. moderate none yes ★★★★ flexicaulis 'Variegata' sulfur-yellow 80 to 100% early September - mid October 54 in. spreading none none no ★★★ gigantea vivid yellow 60 to 80% mid July - late August 40 in. 45 in. minor none no ★★ 'Goldenmosa'• yellow 60 to 80% mid August - mid September 30 in. 26 in. moderate moderate yes ★★★★ 'Goldkind' (Golden Baby) golden yellow 80 to 100% late July - September 28 in. 30 in. minor none no ★★★ graminifolia yellow 40 to 60% mid August - mid September 46 in. spreading none none yes ★★ mollis yellow <20% late July - early September 24 in. spreading minor none yes ★★ ohioensis yellow 80 to 100% mid August - early October 24 in. 20 in. minor none yes ★★ patula yellow 20 to 40% late August - early October 57 in. 30 in. severe none yes ★★ 'Peter Pan'• lemon yellow 80 to 100% mid August - mid September 38 in. 26 in. none minor yes ★★★ riddellii yellow 40 to 60% mid September - late October 33 in. 28 in. minor none yes ★★★★ rigida pale yellow 60 to 80% late August - early October 62 in. 30 in. minor none no ★★ roanensis sulfur-yellow 20 to 40% early September - late October 62 in. 24 in. minor minor yes ★★★ rugosa yellow 60 to 80% early October - early November 94 in. spreading moderate none yes ★★★★ rugosa 'Fireworks' yellow 80 to 100% mid September - late October 54 in. 76 in. minor none no ★★★★ sphacelata 'Golden Fleece' yellow 60 to 80% late August - late October 22 in. 38 in. none none no ★★ squarrosa• yellow 40 to 60% late July - late September 39 in. 24 in. minor none yes ★★ virgaurea 'Praecox'• vivid yellow 60 to 80% late July - late August 50 in. spreading severe moderate yes

Overall Ratings: ★★★★ good, ★★★ fair, ★★ poor, ★ very poor. 1Coverage at peak bloom, approximately 2-3 weeks after first flowers open. 2Powdery mildew level: no injury; minor= < 25% infection; moderate= 26 - 75% infection; severe= > 76% infection. 3Rust level: no injury; minor= < 25% infection; moderate= 26 - 75% infection; severe= > 76% infection. •Plant completed three years of five-year evaluation term Plant Evaluation Notes 3 and terminals of the wiry, zigzagged stems. plant habit, size and floral character to ‘Baby The remainder of the goldenrods received The zigzag goldenrod is rhizomatous, but Sun’ and ‘Goldkind’, but had more powdery poor overall ratings based on higher levels of spreads at a moderate pace. Stems remained mildew and some winter injury. The vase- rust and powdery mildew, severe winter injury, mostly upright to the end of summer. Plants shaped habit remained mostly tight and low flower production and inferior habits. were grown in a partially shaded site that upright, although some loose, open centers In many cases, the typical garden culture was received full sun at noon. were observed as the season progressed. far removed from their native environments Solidago sphacelata ‘Golden Fleece’ is a Flower coverage was lower than the other gar- and more than likely contributed to the compact goldenrod easily recognized by the den hybrids, and the spent flowers reduced the inferior performance. heart-shaped basal leaves topped with airy flo- ornamental impact for the remainder of the The floral display of Solidago ohioensis ral sprays. Tight crowns typified the early-sea- season. Withered leaves on the lower portion was outstanding every year. The large, flat- son habit, but centers often began to open by of the stems were also a problem. Plants were topped clusters of yellow flowers were typi- midsummer. The irregular growth habit was cut to the ground in 1995 only, and a sec- cally at peak display for over two weeks. accentuated by the size differences between ondary bloom was noted in October. Powdery Unfortunately, the lodged stems greatly the basal and stem leaves. The plants began to mildew was observed at 50% or less in all detracted from the overall display, and pow- spread at a modest rate in the third season. The years, and 25% of plants were killed in the dery mildew was a significant problem each floral display was not as impressive as some winter of 1995-96. year. The plants were in good health during other goldenrods, but flowers were produced The arching stems of Solidago caesia the first two years, but only one plant survived into late October and sometimes into tended to be on the weak side, often relaxing the second winter of 1994-95. November. No foliar diseases were observed, to the point of falling. Wreath goldenrod was Low flower production, rust and mildew although chlorosis was a common problem, not bothered by any insects or diseases, but infection, weak stems and winter damage especially on the basal leaves. did suffer some damage in the first winter. ranging from crown injury to death of plants The clump-forming stiff goldenrod, Seedlings were also observed in the later years were the reasons Solidago mollis received a Solidago rigida, had bold-textured, basal of the evaluation. poor rating. Solidago squarrosa, a native of leaves and tall stems with gently arching ter- Solidago gigantea was vigorous, flow- rocky woods, did not flourish in full sun, and it minals. The crosier-like tips of the stems ered well and was fairly resistant to powdery declined throughout the evaluation and even- unfurled to display pale yellow flowers in flat- mildew but received a lower rating due to its tually died during the third season. Weak topped clusters, to 12 inches across. The flow- inferior habit. Although its stems were not the stems and chlorotic or brown leaves con- ers were not as abundant as other species but tallest, they were among the worst for floppi- tributed to an unsightly display. created a pleasant contrast to the gray-green ness. The plants started the summer in good A poor habit, including lodged stems and leaves. Stems remained erect through most of form, but by midseason the crowns opened brown leaves, was characteristic of Solidago the summer, but usually took on a relaxed and all stems eventually lodged on the ground. ‘Goldenmosa’. Moderate levels of powdery appearance by late August. The potential for The stems were cut back each year in late mildew and rust, as well as crown injury in floppiness was noted, although fully flopped summer, resulting in a regeneration of healthy successive winters, weakened the plants until stems were observed in 1996 only. Powdery basal leaves and a small amount of rebloom in all died during the winter of 1995-96. mildew was observed at low levels in all years, September. Although good flower production and healthy, but was not considered an ornamental or Throughout most of the season, the stems well-formed plants typified Solidago ‘Peter health issue. of Solidago riddellii were upright, with Pan’ during the first two years of the evalua- Increased disease problems and/or win- healthy, straplike basal leaves. Despite the fact tion, almost 80% of the plants were killed dur- ter injury contributed to the fair ratings for that 80% of the plants died during the winter ing the winter of 1994-95, and the remaining about one-fourth of the goldenrods. The fine- of 1994-95, the remaining plants were florif- plants were inferior in form and health until textured foliage of Solidago graminifolia erous, with lustrous foliage and upright habits. all died during the following year. formed a billowing mass, and was totally free Mildew was observed at low levels in Solidago altissima was noteworthy for its of rust and powdery mildew. Stems were rou- most years. striking, candelabra-shaped inflorescences, but tinely relaxed-to-floppy, but never to an unfa- The largest goldenrod was Solidago was plagued each year with rust and powdery vorable level. Flower production was fairly rugosa, topping out at almost 8 feet tall. mildew, and severe winter injury in 1994-95. low for the size of the planting, and the lack of Unlike ‘Fireworks’, the species had a spread- It spread vigorously in 1993 and 1994, but was ray florets contributed to a minor floral dis- ing habit with the potential to be invasive. It unable to compete with the adjacent Solidago play. Fifty percent of the plants died during flowered about two to four weeks later than canadensis in later years and was ultimately the winter of 1994-95, but the remaining plants ‘Fireworks’, and the small inflorescences overrun. Solidago canadensis proved to be a rebounded with vigorous growth in the fol- seemed out of scale for a plant of its size. The poor plant in cultivation. Flower production lowing years. By the second year of the evalu- small leaves were burgundy to bronze in was not high, especially for the vigor of the ation the plants had already spread 4 feet into spring and light green in summer; stems were plant; foliage was severely infected with pow- the adjacent plot and turf grass, and in the burgundy too. Powdery mildew was moderate dery mildew and rust each year; and weak fourth year had enveloped a large section of in most years, and the lower leaves began stems and a fast-spreading habit were not the plot, resulting in a mass 8 feet wide and withering early in the summer. The amount of desirable traits for the garden. 27 feet long. stem left bare by withered leaves varied from Severe winter injury reduced the plot of Solidago ‘Crown of Rays’ was similar in one-third to three-fourths of the height. Solidago patula, and severe powdery mildew 4 Plant Evaluation Notes Jenny Lee Jenny Richard Hawke Cathy M.Cathy Jones

Solidago rigida Solidago flexicaulis ‘Variegata’ Solidago ‘Goldkind’ and tattered leaves diminished the ornamental more cultivars and garden hybrids for the gar- Powdery mildew and rust were also sig- display. Lodged stems and low flower produc- dener to choose from, but not all species are nificant diseases that affected the health of tion were also inferior attributes. Solidago roa- adaptable to cultivation or recommended for many goldenrods, although rust was far less nensis was planted in the shady plots because garden use. prevalent or damaging. The goldenrods with it is native to mountain woodlands. Almost Approximately one-fourth of the golden- the highest resistance to powdery mildew 90% of the plants were killed during the win- rods evaluated were determined to be good included Solidago caesia, Solidago flexicaulis ter of 1995-96, and the remaining plants were garden plants with meritorious qualities of ‘Variegata’, Solidago graminifolia, inferior with open crowns, lodged stems, low high flower production, sturdy habits and dis- Solidago ‘Peter Pan’ and Solidago sphacelata flower production, chlorotic leaves and minor ease and pest resistance. Among the best were ‘Golden Fleece’. levels of rust and mildew. Solidago ‘Baby Sun’, Solidago flexicaulis The importance of goldenrods in the late- Declining health due to severe powdery ‘Variegata’, Solidago ‘Goldkind’, Solidago season landscape is indisputable. Gardeners mildew and rust contributed to the ultimate ridiga, Solidago rugosa ‘Fireworks’ and hoping to recreate a bit of nature in their land- demise of Solidago virgaurea ‘Praecox’. Solidago sphacelata ‘Golden Fleece’. All scapes would not be without them. And now Successive years of winter injury resulted in proved to be adaptable to general garden cul- with the newer, shorter cultivars available, complete loss during the third winter. ture and each provided a significant ornamen- more gardeners will be interested in adding a Solidago glomerata, Solidago macrophylla tal display during the growing season. goldenrod for a final flash of color in the and Solidago bicolor were not adaptable to the The native habitat of each species deter- autumn garden. conditions of the test site and died early in the mined whether the plants grew well in culti- Reading List evaluation project. Solidago glomerata vation. The slightly droughty soil and open, declined steadily during its first season and sunny site were not appropriate for all gold- Fernald, M.L. 1989. Gray’s Manual of Botany, Eighth Edition. Portland, Ore.: eventually died in September. Solidago macro- enrods. Some of the species that grew weakly Dioscorides Press. phylla died during the winter of 1994-95, after in cultivation are from native habitats with sig- two years of declining health. Solidago bicol- nificantly different environments from the Lord, T., principal editor. 2000. RHS Plant Finder 2000-2001. London: Dorling or died during the first winter, after a fair dis- evaluation gardens, they include Solidago Kindersley Limited. play in the first season. bicolor (dry, open woods), Solidago macrophylla (damp woods), Solidago ohioensis Phillips, E. and C. C. Burrell. 1993. Rodale’s Summary (bogs, wet prairies and shorelines), Solidago Illustrated Encyclopedia of Perennials. In 1993, at the outset of this project, there Emmaus, Pa.: Rodale Press. patula (wet woods and swamps), Solidago were few cultivars of goldenrod available for roanensis (mountain woods) and Solidago evaluation in our gardens. There were numer- squarrosa (rocky woods). Conversely, ous cultivars listed for sale in Europe and Solidago riddellii, native to swamps and wet many more species offered by native plant meadows, grew fairly well in the test site. nurseries around the country. Today, there are

The Plant Evaluation Program is supported by the Searle Research Endowment, the Woman’s Board of the Chicago Horticultural Society and the Stanley Smith Horticultural Trust. Special thanks to Michael P. Harvey for his help in collecting data in 1995. Plant Evaluation Notes© are periodic publications of the Chicago Botanic Garden. For more information or copies of back issues, contact the Plant Evaluation Program, Chicago Botanic Garden, 1000 Lake Cook Road, Glencoe, Illinois 60022. The Chicago Botanic Garden is owned by the Forest Preserve District of Cook County.